1. Technical Field
The present invention relates to a sheet sucking/feeding device which sucks an uppermost sheet among a plurality of stacked sheets, and separates this uppermost sheet from another sheet therebeneath, and feeds out the uppermost sheet.
2. Description of the Related Art
A technique in which, by using a printing plate (e.g., a PS plate, a thermal plate, a photopolymer plate) in which a recording layer (photosensitive layer) is provided on a support, an image is recorded directly by a laser beam or the like onto the photosensitive layer of the printing plate, has come to be developed as a printing plate exposing device. With this technique, it is possible to quickly record an image onto a printing plate.
In an automatic printing plate exposing device using the technique of recording images onto printing plates, large numbers of printing plates are stacked and accommodated in cassettes. The image forming surface of the printing plate is easily scratched. In order to protect the image forming surface, protective sheets (interleaf sheets) are superposed on the image forming surfaces of the printing plates. Sets of the superposed printing plate and interleaf sheet are successively stacked in layers within the cassette. When a printing plate is to be removed and fed out, one end portion of the uppermost printing plate among the plural printing plates stacked in the cassette is sucked by suction cups so as to separate this printing plate from the others. The printing plates are thereby taken out one-by-one, and are fed sheet-by-sheet (conveyed and fed) to the subsequent process (e.g., an exposure process) while being inverted.
However, when the printing plates are fed out sheet-by-sheet while being sucked by suction cups and taken out one-by-one and inverted as described above, there are cases in which, due static electricity between or sticking due to a vacuum between the uppermost printing plate which the suction cups are sucking and the next printing plate (the printing plate therebeneath), the next printing plate (the printing plate therebeneath) also is lifted up. Therefore, conventionally, a “separating plate” has been provided along the locus of movement along which the printing plate is lifted up and raised while being sucked by the suction cups (e.g., in the corner portion of the top end of the cassette). Due to the printing plate passing by the “separating plate” while contacting the “separating plate” or being temporarily stopped while contacting the “separating plate”, the next printing plate (the printing plate therebeneath) is separated therefrom. Refer to Japanese Patent Application Laid-Open (JP-A) Nos. 2002-128297 and 2001-151360.
Here, at the time the printing plate is vacuum-sucked and conveyed by such suction cups, generally, it is sensed by a pressure sensor that the suction pressure has reached a threshold value which is the necessary suction pressure in all of the processes of “sucking and lifting up”, “separating by the separating plate so as to remove a single printing plate”, and “inverting and conveying”, and then the series of sucking/removing operations is started (refer to JP-A No. 2000-247489). In this case, generally, the necessary suction pressure is greatest at the time of the “separating by the separating plate so as to remove a single printing plate”.
However, when the printing plate directly beneath the suction cups (i.e., the uppermost printing plate) is sucked by the suction cups, the printing plate follows the configurations of the suction cups and is deformed in concave shapes. Therefore, such deformation of the printing plate (the concavity) as described above newly generates a vacuum between that printing plate and the printing plate therebeneath, and there are cases in which the printing plate positioned at the lower side sticks further to the uppermost printing plate (i.e., the uppermost printing plate and the next printing plate (the printing plate therebeneath) stick together due to the vacuum). This sticking together due to the vacuum similarly proceeds successively to the next printing plate (the printing plate beneath). As a result, there are cases in which several printing plates are sucked together in plural layers. Here, even if plural printing plates are sucked together in plural layers in this way, these next printing plates (i.e., the printing plates beneath) can be separated thereafter by the printing plates contacting the “separating plate” as they pass thereby or contacting the “separating plate” while temporarily stopped.
However, in a conventional sucking/removing device using such suction cups, even if plural printing plates are sucked together in plural layers as described above, ultimately, they must be separated such that the printing plates can be conveyed and supplied one-by-one. However, on the other hand, a suction force (suction negative pressure) which is sufficient to suck plural printing plates (which is sufficient to suck plural printing plates together in plural layers) is needed.
Therefore, the surface area sucked by the suction cups (the size or the number of the suction cups), the capacity of the negative pressure source (e.g., a vacuum pump), or the like must be increased and set so as to be sufficient enough to even suck plural printing plates in plural layers together. This is a cause of an increase in the size of the device and an increase in costs.